1. Field of the Invention
This invention relates generally to a flaw detecting apparatus inserted into a cavity of a material being tested, and, more specifically, this invention relates to the utilization of a test apparatus employing ultrasonic waves which is inserted into the bore of a turbine rotor to detect flaws in the material of the rotor.
2. Description of the Prior Art
As turbine sizes have increased over the years in response to the requirements of greater power output, the necessity of improved strength and operational characteristics has created corresponding design problems. Failure of a very large turbine involves not only the considerable expense of repair or replacement, but also frequently results in unacceptable power shortages. Further, the dangers inherent in the forcible failure of a turbine is increased with turbine size. To decrease the chances of turbine failure or shortened operational life, improved materials and better quality control have been utilized.
One of the problems that exists in connection with applying quality control standards to forged turbine rotors is the necessity of determining the existence of flaws that could weaken the rotors and decrease the useful life thereof. Such flaws include items such as cracks, non-metallic inclusions or voids. Various steps have been taken to detect such flaws in a forged turbine rotor, but none of the prior art methods gives the desired reliability, detailed information regarding the flaw, and reproducibility of the test that are required for improved quality control.
For some time, peripheral scanning of the exterior of a rotor by utilization of pulse-echo techniques has been employed. As these tests can be made quite sensitive, they are useful for evaluating initial forging quality. However, the initially forged rotors are considerably altered during the machining cycle, thus making it impossible to duplicate the initial peripheral tests. This makes comparative testing impossible, and hence restricts the utility of this approach. The only surface of the rotor which remains unchanged and accessible to testing is the bore of the rotor. Thus, it is desirable to utilize a testing technique that is adaptable for flaw detection from the bore of the rotor. Although any type of radiation that may be successfully utilized with pulse-echo techniques may be employed, the use of ultrasonic waves seems most desirable.
A non-destructive ultrasonic testing approach utilized for determining flaws in a rotor by testing the bore of the rotor is described by W. R. Marklein and R. E. Warnow in a paper presented to the American Society of Mechanical Engineers at its meeting in New York Nov. 29 - Dec. 4, 1964. In this approach, ultrasonic waves in the longitudinal mode are utilized with a special plexiglass shoe or transducer mount. While this approach does have the advantage of flaw testing from the bore, with the attendant advantages over the external peripheral testing, the results do not give the completeness of information that is desirable for this type of rotor testing.